A modern solid-state laser (SSL) exhibits a high wall-plug efficiency, can operate at high-average power (HAP) levels, and can attain a high beam quality (BQ). Such lasers are highly versatile and are being developed for a variety of applications including air defense and precision strikes, cutting, welding, and drilling in the automotive, aerospace, shipbuilding, and oil and gas industries, and direct material processing such as heat treatment, cutting, and welding.
The lasing medium for SSLs are the atoms of a dopant in a transparent crystalline or amorphous (glass) host material. Physically, the host material can be in various shapes but five principal configurations of an SSL are of interest: a rod, a slab, a disk, a fiber, and a tube.
Average power generated by an SSL is limited principally by thermo-optic distortion (also known as thermal lensing) and thermal stress fracture considerations. Rod lasers are practically limited in these areas, and in practice, rod lasers have been limited in output to about 1 kW of average power. Slab lasers overcome some of the limitations of rod lasers but have poor aperture fill factor (elliptical beam extraction from rectangular aperture). In practice, slab lasers with good beam quality have been limited in output to between about 15 kW and about 20 kW of average power. Disk lasers are functionally very promising as they have very low distortion, but many challenges must be overcome before disk lasers can be scaled to HAP. With regard to a fiber, output of a single fiber is limited to about 1 kW, and hence the output of many fibers must be combined to generate a HAP beam. However, beam-combining technology lags in development. In a tube laser, the SSL gain medium is in the shape of a tube and the laser amplifies an annular beam.
Temperature profiles lead to stress in the laser material. Analyses for uniform heat source density and equal heating power per unit length show that height of the temperature profile in the slab is only 36% of that in the rod, and in the tube it is only 10% of that in the rod. In rod and tube lasers the thermal stress has circular symmetry with the principal stress components being in the azimuthal, radial, and longitudinal directions. The maximum stress at the surface of the rod does not depend on rod diameter. Therefore, the only way of power scaling rod lasers is to use a longer rod or several rods. The surface stress at the slab and tube is proportional to the aspect ratio “b/a”, where “a” is a thickness of the slab or tube, and “b” is the perimeter of the tube or length of cross-section of the slab. Output power is proportional to the lasing medium volume and limited by thermal fracture dependent upon surface stress of the rod, slab, or tube. For tube lasers, output power can be about 10-20 times higher than for rod lasers of equal length. The advantage of the tube geometry is that a much larger aspect ratio b/a can be realized with tubes than with slabs. Therefore, tube SSLs combine many of the attractive properties of the rod, slab, and disk lasers, and overcome many of the aforementioned disadvantages.
However, previous tube SSLs have used flash lamps for excitation, unsophisticated resonator optics, and/or primitive coatings, leading to thermal lensing effects, and birefringence, bifocussing, and alignment problems associated with the tube SSL. Thus, an improved tube SSL that reduces or eliminates thermal lensing effects, and birefringence, bifocussing, and alignment problems is highly desirable.